Characterization of three strains of organophosphorus-resistant cattle ticks (Boophilus microplus) from Bajool, Tully and Ingham

Three new strains of organophosphorus-resistant cattle tick from central and northern Queensland named Bajool, Tully and Ingham are described. In dosage-mortality tests on larvae they all have relatively low levels of resistance but can be distinguished from one another and from other described strains. The levels of acetylcholinesterase activity in the three strains differ from those previously described, and all have enhanced ability to detoxify either coumaphos or chlorpyrifos or both. The Bajool strain came from a property reporting control difficulties with chlorpyrifos. Larvae were found to have a seven times increased resistance to chlorpyrifos, about 14% of the acetylcholinesterase activity present in the Yeerongpilly susceptible strain, and increased ability to detoxify chlorpyrifos. In spraying trials involving all parasitic stages, 0.025 % chlorpyrifos produced mortalities of 92 and > 99 % of the Bajool and Yeerongpilly strains respectively. The Tully strain came from a property using ethion, and the larvae had a six times increased reistance to ethion and coumaphos. The level of acetylcholinesterase activity was about 1.6 times that in the Yeerongpilly strain and the larvae had enhanced ability to detoxify coumaphos. Spraying trials with 0.075 % ethion and 0.025 % coumaphos produced mortalities of about 90 and > 99 % in the Tully and Yeerongpilly strains respectively. The Ingham strain came from a property which had experienced control failures with coumaphos, clenpyrin and chlorpyrifos. The larvae had a four to five times increased resistance to coumaphos and chlorpyrifos, an acetylcholinesterase activity level 1.8 times as great as that of the Yeerongpilly strain, and enhanced detoxication of both chlorpyrifos and coumaphos. Spraying trials with the recommended concentrations of acaricides gave mortalities of 83 % with chlorpyrifos, 78 % with coumaphos and 93 % with ethion compared with > 99% for the Yeerongpilly strain with each of these acaricides.